1. Field of the Invention
The invention relates to charge pumps, and in particular, to an accurate and stable differential charge pump.
2. Related Art
A differential charge pump converts a differential signal into an output voltage by charging a capacitor (or capacitors). FIG. 1 shows a conventional differential charge pump 100 that includes switches S(A), S(B), S(C), and S(D); current sources CS(A), CS(B), CS(C), and CS(D); and a charging capacitor 110.
Switch S(A), current source CS(A), current source CS(B), and switch S(B) are connected in series between supply voltage VDD and ground, thereby forming a first control branch B(A). Similarly, switch S(C), current source CS(C), current source CS(D), and switch S(D) are connected in series between supply voltage VDD and ground, thereby forming a second control branch B(B).
First control branch B(A) includes a terminal T(A) at the junction between current sources CS(A) and CS(B), while the second control branch B(B) includes a terminal T(B) at the junction between current sources CS(C) and CS(D). Capacitor 110 is connected between terminals T(A) and T(B), thereby allowing a differential output voltage VDIFF across capacitor 110 to be read via terminals T(A) and T(B).
Differential charge pump 100 charges or discharges capacitor 110 in response to binary control signals UP and DN. Switches S(A) and S(D) are configured to turn on only when signal UP is asserted HIGH. Meanwhile, switches S(C) and S(B) are configured to turn on only when signal DN is asserted HIGH.
Thus, when signal UP is asserted and signal DN is deasserted, switches S(A) and S(D) are on (closed) and switches S (B) and S (C) are off (open). As a result, current flows through current sources CS(A) and CS(D) while current sources CS(B) and CS(C) are inactive. The current sourced by current source CS(A) charges the plate of capacitor 110 connected to terminal T(A) (i.e., plate 110(A)), while the current sunk by current source CS(D) drains charge from the plate of capacitor 110 connected to terminal T(B) (i.e., plate 110(B)). This charging of plate 110(A) and discharging of plate 110(B) increases output voltage VDIFF.
Likewise, when signal DN is asserted and signal UP is deasserted, switches S(C) and S(B) are closed, while switches S(A) and S(D) are opened. In this case, current sources CS(A) and CS(D) are inactive, while current source CS(C) charges plate 110(B), while current source CS(B) discharges plate 110)(A). The discharging of plate 110(A) and charging of plate 110(B) decreases voltage VDIFF. Thus, differential charge pump 100 increases voltage VDIFF in response to signal UP and decreases voltage VDIFF in response to signal DN.
When differential charge pump 100 is first used, it is generally desirable that plates 110(A) and 110(B) both be at a particular “common mode” voltage. This ensures that the starting output voltage VDIFF is equal to zero. Also, by sizing the common mode voltage to be halfway between supply voltage VDD and ground (i.e., VDD/2), the allowable positive and negative changes in output voltage VDIFF can be maximized.
Unfortunately, when signals UP and DN are the same (i.e., both HIGH or both LOW, the voltage VDIFF across capacitor 110 will remain at whatever voltage was present when control branches B(A) and B(B) were last active. Consequently, each time differential charge pump 100 is used, plates 110(A) and 110(B) must be charged or discharged until they are both at the desired common mode voltage and output voltage VDIFF is set equal to zero. This “calibration” requirement can significantly increase the startup time for any circuit that incorporates differential charge pump 100.
In addition, differential charge pump 100 can also experience a dead zone if the corresponding switches (e.g., switches S(A) and S(D) or switches S(C) and S(B)) don't close at the same time, for example, due to propagation delays. In such circumstances, the terminal associated with the opened switch would be tri-stated, thereby creating a spurious reading of voltage VDIFF.
Accordingly, it is desirable to provide a differential charge pump that maintains a known common-mode voltage and has no dead zone.